Process For The Preparation Of Drospirenone

ABSTRACT

A process is described for the preparation of drospirenone, a synthetic steroid with progestogenic, antimineralocorticoid and antiandrogenic activity, useful for preparing pharmaceutical compositions with contraceptive action; comprising the oxidation of 17α-(3-hydroxypropyl)-6β,7β,15β,16β-dimethylene-5β-androstane-3β,5,17β-triol.

FIELD OF THE INVENTION

The present invention relates to the field of processes for synthesisingsteroids, and in particular to a process for the industrial scalepreparation of drospirenone.

STATE OF THE ART

The compound of formula (I) given hereinafter, whose chemical name is6β,7β;15β,16β-dimethylene-3-oxo-17α-pregn-4-ene-21,17-carbolactone, iscommonly known as drospirenone:

It is a synthetic steroid with progestogenic, antimineralocorticoid andantiandrogenic activity; by virtue of these characteristics drospirenonehas long been used for preparing pharmaceutical compositions withcontraceptive action for oral administration.

Many processes are known in the literature for preparing drospirenone,for example the process described in European Patent No. 0 075 189,starting from 3β,7α,15α-trihydroxy-5-androsten-17-one passing via theintermediate5,6β-epoxy-7β-hydroxy-15β,16β-methylene-3β-pivaloyloxy-5β-androstan-17-one.

As described in EP 0 075 189, this intermediate is then transformed into7α-chloro-5,6β-epoxy-15β,16β-methylene-3β-pivaloyloxy-5β-androstan-17-oneby a reaction that uses tetrachloromethane both as reagent and reactionsolvent. The use of this highly toxic solvent in relatively largequantities is one of the unfavourable aspects of this process.

In the process described in EP 0 075 189 the intermediate17α-(3-hydroxypropyl)-6β,7β;15β,16β-dimethylene-5β-androstane-3β,5,17β-triolis arrived at from the intermediate7α-chloro-5,6β-epoxy-15β,16β-methylene-3β-pivaloyloxy-5β-androstan-17-oneby way of several steps, from which the final product drospirenone isobtained by oxidising with a pyridine/water/chromic anhydride mixtureunder hot conditions. This step constitutes a further disadvantage ofthe known process: chromic anhydride, as all Cr (VI) compounds, isactually a known carcinogen whose use is subject to legislativerestrictions such that the precautions required during the use anddisposal of these products render them practically unusable.

Another process for preparing drospirenone is described in EuropeanPatent No. 0 918 791 B8 wherein the drospirenone is produced in twodistinct phases starting from17α-(3-hydroxypropyl)-6β,7β,15β,16β-dimethylene-5β-androstane-3β,5,17β-triol,using a ruthenium salt as oxidant; in the examples given in said patentcrude drospirenone is obtained with a chromatographic purity of 93%which is then improved by chromatography.

At this point it is worth noting that a possible technique is thesystematic chromatographic purification of industrial batches ofsteroids, requiring however dedicated equipment and working environmentsand consequently a considerable logistic and economic involvement.

There is therefore still a need for a process which enables high puritydrospirenone to be prepared, but without presenting the aforestateddisadvantages of processes of the known art.

SUMMARY OF THE INVENTION

The Applicant has now developed a process that enables drospirenone witha high degree of purity to be obtained, suitable for use in thepreparation of pharmaceutical compositions, and which overcomes theaforestated disadvantages connected to the use of toxic and carcinogenicreagents and the need for chromatographic purifications of crudedrospirenone to obtain a high final purity.

Subject of the present invention is therefore a process for thepreparation of drospirenone, comprising the oxidation of17α-(3-hydroxypropyl)-6β,7β,15β,16β-dimethylene-5β-androstane-3β,5,17β-triolof formula (VIII) with a suitable oxidising agent in an organic solventin the presence of a catalytic amount of2,2,6,6-tetramethylpiperidine-1-oxyl radical or a derivative thereof,said oxidation being followed by the addition of a protic acid directlyinto the same reactor in which the oxidation took place, to obtain thedrospirenone of formula (I)

Further subject of the invention is drospirenone obtained by the abovesaid process, and a pharmaceutical composition comprising thedrospirenone obtained by the above said process as active principle, anda carrier.

The characteristics and advantages of the present process will beillustrated in detail in the description which follows.

DETAILED DESCRIPTION OF THE INVENTION

The oxidation substrate of the present process, i.e.17α-(3-hydroxypropyl)-6β,7β,15β,16β-dimethylene-5β-androstane-3β,5,17β-triol,can be obtained starting from commercial products by procedures known toany expert of the art. Preferably this product is obtained from5,6β-epoxy-7β-hydroxy-15β,16β-methylene-3β-pivaloyloxy-5β-androstan-17-one,in accordance with the procedure comprising the following steps:

a) bromination in position 7α of5,6β-epoxy-7β-hydroxy-15β,16β-methylene-3β-pivaloyloxy-5β-androstan-17-oneof formula (II) to obtain7α-bromo-5,6β-epoxy-15β,16β-methylene-3β-pivaloyloxy-5β-androstan-17-oneof formula (III) by reacting the compound of formula (II) with mesylchloride to obtain the corresponding mesylate which is not isolated andfrom which the compound of formula (III) is obtained by the addition oflithium bromide:

in which the symbol PV indicates a pivaloyl group, i.e. atrimethylacetylgroup;b) opening the epoxy ring and removing the bromine from7α-bromo-5,6β-epoxy-15β,16β-methylene-3β-pivaloyloxy-5β-androstan-17-oneof formula (III) coming from step a) to obtain5-hydroxy-15β,16β-methylene-3β-pivaloyloxy-5β-androst-6-en-17-one offormula (IV):

c) hydrolysis of the pivaloyl group of5-hydroxy-15β,16β-methylene-3-pivaloyloxy-5β-androst-6-en-17-one offormula (IV) coming from step b) to obtain3β,5-dihydroxy-15β,16β-methylene-5β-androst-6-en-17-one of formula (V):

in which PV is defined as above,d) methylenation at the Δ⁶ double bond of3β,5-dihydroxy-15β,16β-methylene-5β-androst-6-en-17-one of formula (V)coming from step c), to obtain3β,5-dihydroxy-6β,7β;15β,16β-dimethylene-5β-androst-17-one of formula(VI)

e) reacting 3β,5-dihydroxy-6β,7β;15β,16β-dimethylene-5β-androst-17-oneof formula (VI) coming from step d) with propargyl alcohol to obtain17α-(3-hydroxy-1-propinyl)-6β,7β;15β,16β-dimethylene-5β-androstane-3,5,17β-triolof formula (VII)

f) hydrogenating17α-(3-hydroxy-1-propinyl)-6β,7β;15β,16β-dimethylene-5β-androstane-3β,5,17β-triolof formula (VII) coming from step e) to obtain17α-(3-hydroxypropyl)-6β,7β;15β,16β-dimethylene-5β-androstane-3β,5,17β-triolof formula (VIII)

The starting5,6β-epoxy-7β-hydroxy-15β,16β-methylene-3β-pivaloyloxy-5β-androstan-17-oneof formula (I) can be in its turn obtained from3β-hydroxy-5-androsten-17-one as described in European Patent No. 0 075189.

The bromination reaction in step a) is preferably carried out by addingmesyl chloride and pyridine to the starting compound at room temperaturewith the formation of the corresponding mesylate, then adding lithiumbromide dissolved in water and bringing the temperature to valuesbetween 70 and 75° C.

The successive steps a) to f) can be carried out in accordance withprocedures commonly utilised and known to any skilled person.

The term “suitable oxidising agent” in accordance with the inventionmeans a product chosen from the group consisting of hypohalides ofalkali and alkaline-earth metals, preferably calcium and sodiumhypochlorite, iodine, oxygen in the presence of CuCl, potassiumperoxymonosulfate KHSO₅ known commercially as Oxone®, and1,3,5-trichloro-2,4,6-triazinetrione.

Derivatives of the 2,2,6,6-tetramethylpiperidine-1-oxyl radical ofpossible use in the present process are chosen for example from the4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl radical, the4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl radical and the4-(benzoyloxy)-2,2,6,6-tetramethylpiperidine-1-oxyl radical. As organicsolvent for the oxidation reaction a solvent chosen from the groupconsisting of ethers such as acetone, methyl t-butyl ether andtetrahydrofuran, esters such as ethyl acetate, hydrocarbons such astoluene, halogenated hydrocarbons, such as methylene chloride, andmixtures thereof, can be used. The oxidation reaction and subsequentdehydration can be carried out for example at a temperature between 0and 40° C., preferably at a temperature between 20 and 25° C.

Preferred reaction conditions are those in which the oxidation iscarried out with calcium hypochlorite using as organic solvent amethylene chloride/tetrahydrofuran mixture, preferably in a 8.5/1 ratio,at a temperature between 20 and 25° C. in the presence of a catalyticamount of 2,2,6,6-tetramethylpiperidine-1-oxyl radical and in thepresence of an aqueous sodium bicarbonate solution.

At the end of the oxidation reaction a protic acid is added directly tothe organic solution in which the oxidation reaction took place.Alternatively, the organic solution in which the oxidation reaction tookplace is distilled until a semi-solid residue is obtained which is thenredissolved in a suitable organic solvent, and to the so obtainedsolution the protic acid is then added.

The aforesaid protic acid is chosen for example from the groupconsisting of concentrated hydrochloric acid, dilute hydrochloric acidand p-toluenesulfonic acid; preferably the protic acid used isp-toluenesulfonic acid monohydrate.

The crude drospirenone obtained with the present process as describedabove has a high degree of purity, being greater than 96.5%, which cannevertheless be increased by subjecting the crude product coming fromthe oxidation to a purification procedure to obtain drospirenone with adegree of purity greater than 99.5%.

To obtain drospirenone with said degree of purity no chromatographicprocedure is necessary, but a filtration through gel and decolourisingcarbon is sufficient, followed by crystallisation of the filtrate fromsolvent, the two steps of filtration and crystallisation possibly beingrepeated one or more times.

Preferably the gel utilised in accordance with the invention is silicagel, while the crystallisation solvent can be chosen from the groupconsisting of ethyl ether, isopropyl ether, ethyl acetate, methyltertbutyl ether, isopropyl acetate, methyl acetate, dimethoxyethane,methanol, ethanol, isopropanol, methylene chloride, acetone,dimethylacetamide, dimethylformamide and mixtures thereof; the preferredcrystallisation solvent is isopropyl acetate.

In accordance with a particularly preferred embodiment of the invention,the present purification procedure comprises the following steps:

i) dissolving crude drospirenone in a suitable organic solvent, furthercontaining silica gel and decolourising carbon, and filtering thesolution thus obtained;ii) distilling the solution coming from step i) and redissolving thedistillate in a second organic solvent;iii) distilling the solution coming from step ii) and redissolving thedistillate in said second organic solvent;iv) crystallising pure drospirenone from the solution coming from stepiii);v) recovering pure drospirenone by filtering, washing over the filter atleast once with a suitable organic solvent, then drying at a pressurelower than atmospheric pressure;vi) if necessary repeating steps i) to v), starting from thedrospirenone coming from step v).

The amount of silica gel and decolourising carbon employed in step i) ispreferably less than 5% by weight with respect of the weight of thecrude drospirenone to be purified.

The distillation steps ii) and iii) are preferably carried out at adistillation temperature between 35 and 45° C., and at a pressure lowerthan atmospheric pressure.

In step iv) said crystallisation is carried out at a temperature between0 and 5° C. for a time period between 60 and 180 minutes.

The organic solvent used in steps i), ii), iii) and v) is chosen forexample from the group consisting of ethyl ether, isopropyl ether, ethylacetate, isopropyl acetate, methyl acetate, dimethoxyethane, methanol,ethanol, isopropanol, methylene chloride, acetone, dimethylacetamide,dimethylformamide, methyl tertbutyl ether and mixtures thereof.

Preferably the organic solvent in step i) is methylene chloride, theorganic solvent in step ii) is isopropyl acetate, and in step v) twowashings are undertaken, the first with isopropyl acetate and the secondwith ethyl ether.

The present process for drospirenone preparation as described above hasproved to be advantageous in that it enables preparation of theintermediate7α-bromo-5,6β-epoxy-15β,16β-methylene-3β-pivaloyloxy-5β-androstan-17-one,useful for drospirenone synthesis, while avoiding toxic solvents andreagents such as tetrachloromethane as used in the process given in EP 0075 189. Furthermore, though preparation of this brominated intermediatepasses via the formation of a mesylated intermediate, it does notinvolve an additional process step because the mesylate is not isolatedbut brominated directly.

The use of carcinogenic reagents is also avoided in the oxidation stepwhich, as well as not requiring carcinogenic reagents, is just asefficient as the oxidation with chromic anhydride described in EP 0 075189.

Finally, the purification process described above enables the invertedlactone fraction that is present in the crude product and identified asZK35096 in U.S. Pat. No. 6,121,465, to be completely eliminated withoutthe use of chromatographic techniques. This purification process isapplicable and useful for the purification not only of drospirenoneprepared in accordance with the present process, but also of productsobtained with other processes and in which the aforementioned invertedlactone is present as impurity.

The following examples are given as non-limiting illustrations of thepresent invention.

EXAMPLE 1 Preparation of7α-bromo-5,6β-epoxy-15β,16β-methylene-3β-pivaloyloxy-5β-androstan-17-oneStep a)

67.5 g of5,6β-epoxy-7β-hydroxy-15β,16β-methylene-3β-pivaloyloxy-5β-androstan-17-oneare dissolved in 205 ml of pyridine in a 2 litre flask, under nitrogen.

17.5 ml of mesyl chloride are added from a dropping funnel, maintaininga temperature of 20/25° C.

The mixture is stirred for 1 hour at 20° C. to obtain a thick orangesuspension.

The progress of the reaction is checked by TLC. Once the reaction iscompleted, 83.2 g of lithium bromide dissolved in 54 ml of water areadded and the temperature is brought to 70/75° C. After 3 hours another8 g of lithium bromide dissolved in water and 50 ml of pyridine areadded.

At the end of the reaction (checked by TLC) the temperature is broughtto 60° C. and 700 ml of water are added; it is left to cool to 15/20°C., maintaining under stirring for 1 hour at this temperature.

The solid is filtered off and washed with 500 ml of water.

The solid is dried for 24 hours under reduced pressure at 45° C. toobtain 69.5 g of the title compound.

On the product thus obtained, purified by chromatography, ¹H-NMR andmass spectroscopic analyses were carried out, and the following resultswere obtained:

¹H-NMR (300 MHz, CDCl₃): δ (ppm) 0.92 (18-Me, s, 3H); 1.04 (19-Me, s,3H); 1.08-1.16 (m, 1H); 1.16 (t-But, s, 9H); 1.18-1.28 (m, 1H);1.36-1.60 (m, 8H); 1.62-1.68 (m, 1H); 1.72-1.76 (m, 1H); 1.84-1.96 (m,3H); 2.04-2.16 (m, 3H); 3.46 (6-H, broad s, 1H); 4.73 (7-H, broad s,1H); 4.76-4.84 (3-H, m, 1H).

Electron impact mass spectroscopy: m/z [376] and [378]=M⁺-C(CH₃)₃—COOH;[297] and [299]=M⁺-C(CH₃)₃—COOH—Br

EXAMPLE 2 Preparation of5-hydroxy-15β,16β-methylene-3β-pivaloyloxy-5β-androst-6-en-17-one Stepb)

27 g of powdered zinc suspended in 91 ml of THF (tetrahydrofuran) arefed into a 1 litre flask, under nitrogen.

A solution of 67.5 g of7α-bromo-5,6β-epoxy-15β,16β-methylene-3β-pivaloyloxy-5β-androstan-17-one,prepared as described in Example 1, in 277 ml of THF is then added; 19.9ml of glacial acetic acid are slowly added dropwise, maintaining thetemperature below 60° C. during the addition. The reaction mixture ismaintained under stirring for 3 hours at 59/60° C.

At the end of the reaction (checked by TLC) and after cooling to 50° C.,the zinc is filtered off over dicalite and the filter washed with 200 mlof THF.

The filtered solution is brought to pH 9 with 60 ml of triethylamine.

The solution is concentrated under reduced pressure at 50° C. to obtainabout 180 g of a semi-solid product which is dissolved in 500 ml of a 5%acetic acid-water solution (pH=4 with a precipitate).

It is maintained under stirring for 1 hour at 10/15° C., the solid isfiltered off and washed with 500 ml of water then dried under reducedpressure for 12 hours at 50° C., thus obtaining 57 g of crude product.

The crude product is refluxed for 1 hour in a mixture of 115 ml oft-butyl methyl ether and 114 ml of ethyl acetate (partial dissolution).

It is cooled for 1 hour at 0/5° C., the solid is filtered off and washedwith t-butyl methyl ether and dried under reduced pressure for 1 hour at60° C.

44.6 g of the title compound are obtained.

The analytical data obtained from a sample purified by chromatographycorrespond to those given in EP 0 075 189.

EXAMPLE 3 Preparation of3β,5-dihydroxy-15β-16β-methylene-5β-androst-6-en-17-one Step c)

43 g of5-hydroxy-15β,16β-methylene-3β-pivaloyloxy-5β-androst-6-en-17-oneprepared as described above in Example 2, 430 ml of THF, 215 ml ofmethanol and 12.9 g of potassium hydroxide are fed into a 2 litre flask,under nitrogen at 20° C. The suspension is stirred at 20° C. for 3hours.

At the end of the reaction (checked by TLC), the reaction mixture ispoured into 2 litres of water, brought to pH 7 with 20% sulphuric acid(about 25 ml) then the suspension is stirred for 1 hour at 0/5° C. Thesolid is filtered off, washed with water and dried for 12 hours underreduced pressure at 50° C. to obtain 30.6 g of the title compound.

The analytical data obtained for a sample purified by chromatographycorrespond to those given in EP 0 075 189.

EXAMPLE 4 Preparation of3β,5-dihydroxy-6β,7β;15β,16β-dimethylene-5β-androst-17-one Step d)

29 g of 3β,5-dihydroxy-15β,16β-methylene-5β-androst-6-en-17-one preparedas described above in Example 3 are fed into a 2 litre flask undernitrogen at 20° C. with 410 ml of THF.

0.6 g of copper (II) acetate hydrate are added and the mixture ismaintained under stirring until the solution is clear (green).

37.9 g of finely powered zinc are added and, after stirring for 15minutes, 1.7 ml of acetic acid are further added.

The mixture is further stirred for 30 minutes at 20° C. then heated to50° C., 32.3 ml of methylene bromide are added and it is refluxed for 2hours.

At the end of the reaction (checked by TLC) it is cooled to 20° C. and amixture consisting of 26.8 ml acetic acid in 450 ml water is addedslowly while cooling.

The mixture is filtered through dicalite and the panel is washed with600 ml of toluene.

The phases are separated and the aqueous phase is extracted with 200 mlof toluene. The joined organic phases are washed with 350 ml of water.

The organic phase is dried over sodium sulphate, filtered andconcentrated under reduced pressure at 60° C. until a solid is obtained.

The solid is dissolved with 50 ml of a 3/1 heptane/ethyl acetate mixtureand filtered off, then dried for 12 hours under reduced pressure at 45°C. to obtain 25.5 g of the title compound.

The analytical data obtained from a sample purified by chromatographycorrespond to those given in EP 0 075 189.

EXAMPLE 5 Preparation of17α-(3-hydroxy-1-propinyl)-6β,7β;15β,16β-dimethylene-5β-androstane-3β,5,17β-triolStep e)

24 g of 3β,5-dihydroxy-6β,7β;15β,16β-dimethylene-5β-androst-17-oneprepared as described above in Example 4 are fed into a 1 litre flask,under nitrogen at 20° C., with 480 ml THF.

The mixture is cooled to 0/5° C. and 72 g of potassium methylate areadded (yellow suspension).

While maintaining the temperature at 0/5° C. 48 ml of propargyl alcoholdiluted with 90 ml of THF are added slowly (thick orange solution).

A further 150 ml of THF are added when the solution density rendersstirring impossible. The solution is maintained under stirring for 12hours at 0/5° C.

At the end of the reaction (checked by TLC) the very thick suspension ispoured into 2 litres of water and ice (an orange solid precipitates).

The solid obtained is extracted with 1.5 litres of isopropyl acetate.

The organic phase is dried over sodium sulphate, filtered andconcentrated under reduced pressure at 50° C. to obtain a solid.

The solid is filtered off from heptane and dried for 12 hours at 45° C.under reduced pressure to obtain 27.1 g of the title compound.

The analytical data obtained from a sample purified by chromatographycorrespond to those given in EP 0 075 189.

EXAMPLE 6 Preparation of17α-(3-hydroxypropyl)-6β,7β;15β,16β-dimethylene-5β-androstane-3β,5,17β-triolStep f)

A solution of 25.1 g17α-(3-hydroxy-1-propinyl)-6β,7β;15β,16β-dimethylene-5β-androstane-3β,5,17β-triolprepared as described above in Example 5, in 930 ml of a mixtureprepared with 750 ml of THF, 375 ml of methanol and 1.5 ml of pyridineis fed into an autoclave.

5 g of 5% Pd/C catalyst are added and hydrogenation is carried out atatmospheric pressure (20/25° C.) for 2 hours.

At the end of the reaction (checked by TLC) the suspension is filteredthrough dicalite then the filter is washed with methylene chloride.

The product is concentrated under reduced pressure at 50° C. to obtain32 g of the title compound.

The crude title product contained small quantities of the two6β,7β;15β,16β-dimethylene-3β,5β-dihydroxy-17α-pregn-21,17-carbolactols.It was nevertheless advantageously used for the subsequent reaction,without any further purification. A sample of the title product purifiedby chromatography gave the following results with ¹H-NMR analysis:

¹H-NMR (300 MHz, CDCl₃): δ (ppm) 0.84 (18-Me, s, 3H); 0.88 (19-Me, s,3H); 1.72 (s, —OH); 2.32-2.40 (m, —OH); 2.6 (s, —OH); 3.38-3.40 (m,—OH); 3.64-3.76 (—CH₂OH, m, 2H); 4.0 (3-H, m, 1H).

The signals of the hydroxyl protons were identified by deuteration.

The crude reaction product used for the subsequent reaction alsopresented the following signals:

¹H-NMR (300 MHz, CDCl₃): δ (ppm) 5.50 (17-O—CHOH-21, t, 1H); 5.58(17-O—CHOH-21, t, 1H).

EXAMPLE 7 Preparation of6β,7β;15β,16β-dimethylene-3-oxo-17α-pregn-4-en-21,17-carbolactone(DROSPIRENONE) Oxidation

50 g of17α-(3-hydroxypropyl)-6β,7β;15β,16β-dimethylene-5β-androstane-3β,5,17β-triolprepared as described above in Example 6, 850 ml of methylene chlorideand 100 ml of THF are fed into a reactor, and stirred at a temperatureof 20° C.

A solution, prepared by dissolving 75 g of sodium bicarbonate in 750 mlof water, is added to the organic solution thus obtained.

While maintaining the biphasic solution under vigorous stirring at 20°C., 1.2 g of 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) and 35g of calcium hypochlorite are added in portions, while monitoringoxidation reaction progress by TLC.

The biphasic solution is filtered, the two phases are left to separate,and the organic phase is washed first with an aqueous sodium bisulfatemonohydrate solution then with water.

The organic phase is concentrated at 40° C. under vacuum until asemi-solid residue is obtained, which is then dissolved with 560 ml THF;4.9 g of p-toluenesulfonic acid monohydrate are added to the solutionthus obtained and maintained under stirring for 1 hour at 20° C., whilemonitoring the formation of drospirenone by means of TLC.

Once the reaction is completed the product is neutralised with anaqueous 10% sodium bicarbonate solution and extracted with 800 ml ofisopropyl acetate. The organic phase is washed with water andconcentrated under vacuum at 40° C.

The residue is firstly dissolved with isopropyl acetate thenconcentrated again under vacuum at 40° C. and dissolved once more withisopropyl acetate at 0/5° C., to obtain a suspension.

By filtering this suspension, washing the solid with ethyl ether anddrying it under vacuum at 40° C., 31.3 g of crude drospirenone areobtained which are then fed into a container with 150 ml of methylenechloride. 2 g of decolourising carbon and 1.45 g of silica gel are thenadded. The suspension is then filtered and concentrated to a smallvolume by distillation under vacuum at 40° C.

The residue is then dissolved with isopropyl acetate, concentrated to asmall volume by distillation under vacuum at 40° C., again dissolvedwith 25 ml of isopropyl acetate and maintained under stirring at 30° C.for 15 minutes, then at 0/2° C. for 2 hours.

After filtering, the solid obtained is washed first with cold isopropylacetate then with ethyl ether. After drying under vacuum at 40° C. untila constant weight is achieved, 28.9 g of drospirenone are obtained whoseanalytical data correspond with those given in the literature.

EXAMPLE 8 Preparation of6β,7β;15β,16β-dimethylene-3-oxo-17α-pregn-4-en-21,17-carbolactone(DROSPIRENONE) Oxidation

12 g of17α-(3-hydroxypropyl)-6β,7β,15β,16β-dimethylene-5β-androstane-3β,5,17β-triolprepared as described above in Example 6, 170 ml of methylene chlorideand 20 ml of THF are fed into a reactor. The mixture is stirred at 20°C. until a homogeneous solution is obtained.

A solution, prepared by dissolving 15 g of sodium bicarbonate in 150 mlof water, is added to the organic solution thus obtained.

While maintaining the biphasic solution under vigorous stirring at 20°C., 0.54 g of 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) and8.6 g of calcium hypochlorite are added in portions, while monitoringoxidation reaction progress by TLC.

On completion of the oxidation, the biphasic solution is filtered andthe two phases are left to separate. 1.5 g of p-toluenesulfonic acidmonohydrate are added to the organic phase.

The mixture is maintained under stirring for about 3 hours at 20° C.,while monitoring the reaction by TLC.

When the reaction is complete, neutralisation is carried out with an 1%aqueous sodium bicarbonate solution.

The reaction proceeds as described above in Example 7 to finally obtain6.5 g of drospirenone whose analytical data correspond to those given inthe literature and those obtained for the product in Example 7.

1. Process for the preparation of drospirenone, comprising the oxidationof17α-(3-hydroxypropyl)-6β,7β,15β,16β-dimethylene-5β-androstane-3β,5,17β-triolof formula (VIII) with a suitable oxidising agent in an organic solventin the presence of a catalytic amount of the2,2,6,6-tetramethylpiperidine-1-oxyl radical or a derivative thereof,said oxidation being followed by the addition of a protic acid directlyinto the same container in which the oxidation took place, to obtain thedrospirenone of formula (I)


2. The process according to claim 1, wherein said17α-(3-hydroxypropyl)-6β,7β,15β,16β-dimethylene-5β-androstane-3β,5,17β-triolof formula (VIII) is prepared starting from5,6β-epoxy-7β-hydroxy-15β,16β-methylene-3β-pivaloyloxy-5β-androstan-17-oneof formula (II) in accordance with the following steps: a) brominationin position 7α of5,6β-epoxy-7β-hydroxy-15β,16β-methylene-3β-pivaloyloxy-5β-androstan-17-oneof formula (II) to obtain7α-bromo-5,6β-epoxy-15β,16β-methylene-3-pivaloyloxy-5β-androstan-17-oneof formula (III) by reacting the compound of formula (II) with mesylchloride to obtain the corresponding mesylate which is not isolated andfrom which the compound of formula (III) is obtained by adding lithiumbromide:

in which the symbol PV indicates a pivaloyl group, i.e. atrimethylacetyl group; b) opening the epoxy ring and removing thebromine from7α-bromo-5,6β-epoxy-15β,16β-methylene-3β-pivaloyloxy-5β-androstan-17-oneof formula (III) derived from step a) to obtain the5-hydroxy-15β,16β-methylene-3β-pivaloyloxy-5β-androst-6-en-17-one offormula (IV):

c) hydrolysis of the pivaloyl group of5-hydroxy-15β,16β-methylene-3-pivaloyloxy-5β-androst-6-en-17-one offormula (IV) coming from step b) to obtain the3β,5-dihydroxy-15β,16β-methylene-5β-androst-6-en-17-one of formula (V):

in which PV is defined as above, d) methylenation at the Δ⁶ double bondof 3β,5-dihydroxy-15β,16β-methylene-5β-androst-6-en-17-one of formula(V) coming from step c) to obtain the3β,5-dihydroxy-6β,7β;15β,16β-dimethylene-5β-androst-17-one of formula(VI)

e) reacting the3β,5-dihydroxy-6β,7β;15β,16β-dimethylene-5β-androst-17-one of formula(VI) coming from step d) with propargyl alcohol to obtain the17α-(3-hydroxy-1-propinyl)-6β,7β;15β,16β-dimethylene-5β-androstane-3β,5,17β-triolof formula (VII)

f) hydrogenating the17α-(3-hydroxy-1-propinyl)-6β,7β;15β,16β-dimethylene-5β-androstane-3β,5,17β-triolof formula (VII) coming from step e) to obtain the17α-(3-hydroxypropyl)-6β,7β;15β,16β-dimethylene-5β-androstane-3β,5,17β-triolof formula (VIII)


3. The process according to claim 1, wherein said oxidising agent isselected from the group consisting of alkali metal and alkaline-earthmetal hypohalides, iodine, oxygen in the presence of CuCl, potassiumperoxymonosulphate and 1,3,5-trichloro-2,4,6-triazinetrione.
 4. Theprocess according to claim 3, wherein said oxidising agent is selectedfrom sodium hypochlorite and calcium hypochlorite.
 5. The processaccording to claim 1, wherein said derivative of the2,2,6,6-tetramethylpiperidine-1-oxyl radical is selected from the groupconsisting of the 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxylradical, the 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl radical andthe 4-(benzoyloxy)-2,2,6,6-tetramethylpiperidine-1-oxyl radical.
 6. Theprocess according to claim 1, wherein said organic solvent is selectedfrom the group consisting of ethers, esters, hydrocarbons, halogenatedhydrocarbons and mixtures thereof.
 7. The process according to claim 1,wherein said organic solvent is selected from the group consisting ofacetone, toluene, methyl t-butyl ether, ethyl acetate, methylenechloride, tetrahydrofuran and mixtures thereof.
 8. The process accordingto claim 1, wherein said oxidation is carried out at a temperaturebetween 0 and 40° C.
 9. The process according to claim 1, wherein saidoxidation is carried out with calcium hypochlorite using as organicsolvent a methylene chloride/tetrahydrofuran mixture at a temperaturebetween 20 and 25° C. in the presence of a catalytic amount of the2,2,6,6-tetramethylpiperidine-1-oxyl radical and in the presence of anaqueous sodium bicarbonate solution.
 10. The process according to claim9, wherein said methylene chloride/tetrahydrofuran mixture is used in aratio of 8.5/1.
 11. The process according to claim 1, wherein saidprotic acid is selected from the group consisting of concentratedhydrochloric acid, dilute hydrochloric acid and p-toluenesulfonic acid.12. The process according to claim 11, wherein said protic acid isp-toluenesulfonic acid monohydrate.
 13. The process according to claim1, wherein said protic acid addition is carried out at a temperaturebetween 0 and 40° C.
 14. The process according to claim 13, wherein saidprotic acid addition is carried out at a temperature between 20 and 25°C.
 15. The process according to claim 1, wherein said protic acid isadded directly to the organic solution in which the oxidation reactiontook place.
 16. The process according to claim 1, wherein said proticacid is added to the solution obtained by dissolving in a suitableorganic solvent the semi-solid residue coming from the distillation ofthe organic solution in which the oxidation reaction took place.
 17. Theprocess according to claim 2, wherein said bromination reaction in stepa) is carried out by adding mesyl chloride and pyridine to the startingcompound at room temperature with the formation of the correspondingmesylate, then adding lithium bromide dissolved in water and bringingthe temperature to values between 70 and 75° C.
 18. The processaccording to claim 1, further comprising purification of the crudedrospirenone by a procedure comprising gel filtration and filtratecrystallisation from organic solvent, said procedure possibly beingrepeated one or more times.
 19. The process according to claim 18,wherein said purification comprises the following steps: i) dissolvingcrude drospirenone in a suitable organic solvent, further containingsilica gel and decolourising carbon, and filtering the solution thusobtained; ii) distilling the solution coming from step i) andredissolving the distillate in a second organic solvent; iii) distillingthe solution coming from step ii) and redissolving the distillate insaid second organic solvent; iv) crystallising pure drospirenone fromthe solution coming from step iii); v) recovering pure drospirenone byfiltering, washing over the filter at least once with a suitable organicsolvent, and drying at a pressure lower than atmospheric pressure; vi)optionally repeating steps i) to v), starting from the drospirenonecoming from step v).
 20. The process according to claim 19, wherein theamount of silica gel and decolourising carbon utilised is less than 5%by weight with respect to the weight of the crude drospirenone to bepurified.
 21. The process according to claim 19, wherein thedistillation temperature in steps ii) and iii) is between 35 and 45° C.22. The process according to claim 19, wherein said distillation insteps ii) and iii) is carried out at a pressure lower than atmosphericpressure.
 23. The process according to claim 19, wherein saidcrystallisation in step iv) is carried out at a temperature between 0and 50° C. for a time period between 60 and 180 minutes.
 24. The processaccording to claim 19, wherein said organic solvent in steps i), ii),iii) and v) is selected from the group consisting of ethyl ether,isopropyl ether, ethyl acetate, methyl tertbutyl ether, isopropylacetate, methyl acetate, dimethoxyethane, methanol, ethanol,isopropanol, methylene chloride, acetone, dimethylacetamide,dimethylformamide and mixtures thereof.
 25. The process according toclaim 19, wherein said organic solvent in step i) is methylene chloride,said organic solvent in step ii) is isopropyl acetate, and in step v)two washings are carried out, the first with isopropyl acetate and thesecond with ethyl ether.
 26. Drospirenone having purity higher than96.5%, obtained by oxidation of compound of formula (VIII) as defined inclaim 1, in an organic solvent, in the presence of a catalytic amount of2,2,6,6-tetramethylpiperidine-1-oxyl radical or a derivative thereof,and addition of aprotic acid.
 27. Drospirenone having purity higher than99.5%, obtained by purification of the product obtained by the processas defined in claim 1, by gel filtration and filtrate crystallisationfrom organic solvent, without subjecting the product to anychromatographic procedure.
 28. A pharmaceutical composition comprisingdrospirenone as defined in claim 26 as active principle, and a carrier.29. A pharmaceutical composition comprising drospirenone as defined inclaim 27 as active principle, and a carrier.